Integrated Digital Discriminator For a Silicon Photomultiplier

ABSTRACT

Apparatuses and methods are provided that minimize the effects of dark-current pulses. For example, in one embodiment of the invention, a method is provided where a first pixel is struck (i.e., a primary pixel). Pixels struck within a fixed time frame after the primary pixel is struck are referred to as secondary pixels. After a short fixed time frame has expired, the number of primary and secondary pixels is added. If the count exceeds a threshold, the primary pixel was activated by the first (or early) photon from a true gamma event. If the threshold is not met then it is likely the primary pixel generated a dark pulse that should be ignored.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 13/556,331 filed on Jul. 24, 2012, which is hereby incorporatedby reference in its entirety.

BACKGROUND

1. Field of the Invention

Embodiments of the present invention generally relate to analog anddigital discriminators, and more specifically to apparatuses and methodsfor minimizing the effects of dark-current pulses from SiliconPhotomultipliers.

2. Description of the Related Art

Nuclear medicine is a unique medical specialty wherein radiation is usedto acquire images that show the function and anatomy of organs, bonesand/or tissues of the body. Radiopharmaceuticals are introduced into thebody, either by injection or ingestion, and are attracted to specificorgans, bones and/or tissues of interest. For example, theradiopharmaceutical (e.g., rubidium) is injected into the bloodstream.

The radiopharmaceutical produces gamma photon emissions that emanatefrom the body. One or more detectors are used to detect the emittedgamma photons and the information collected from the detector(s) isprocessed to calculate the position of origin of the emitted photon fromthe source (i.e., the body organ or tissue under study). Theaccumulation of a large number of events (e.g., a single gamma whenusing Single Photon Emission Computed Tomography (“SPECT”) andcoincident gamma events when using Positron Emission Tomography (“PET”))allows an image of the organ or tissue under study to be displayed.

FIG. 1 depicts a prior art system 100 that includes a known apparatus102 for superposed magnetic resonance (“MR”) and PET imaging. Theapparatus 102 includes a known MR tube 104. The MR tube 104 defines alongitudinal direction Z (not shown), parallel to a longitudinal axis ofa patient (also not shown), which extends orthogonally with respect tothe plane of the drawing in FIG. 1.

As shown in FIG. 1, a plurality of PET detector units 106 arranged inpairs opposite each other about the longitudinal direction z is arrangedcoaxially within the PET scanner 104. The PET detector units 106preferably include a silicon photo-multiplier (“SiPM”) array 108.

A computer 120 is also included in the system 100. The computer 120includes a central processing unit (“CPU”) 114 for image processing ofsuperimposed MR and PET images, a user interface 118 (depicted as akeyboard), and a monitor 116 for viewing input and output data.

The prior art SiPM array 108 is depicted in FIG. 2 and consists of ann×n array of pixels 202. For illustrative purposes the n×n pixel array202 is depicted as a 5×5 pixel array. The Pixels in the array 202 areidentified by an (x, y) index. For example, pixels along the “X” axisare denoted as 202(1,1), 202(1,2), . . . , and 202(1,5); and pixelsalong the “Y” axis are denoted as 202(2,1), 202(2,1), . . . , and202(5,1).

Typically, each individual pixel is an m×m array of parallel microcells204, with each microcell consisting of a reverse biased avalanchephotodiodes (“APD”) 206 ₁, . . . , 206 _(m) and an active or passivequenching mechanism such as resistors 208 ₁, . . . , 208 _(m)(collectively 206 and 208, respectively). Parasitic capacitance acrossthe quench resistor 208 is also present.

The APD 206 is reverse biased to a voltage V_(b), greater than thebreakdown voltage V_(brk). When a photon is absorbed in the junction, itcan cause a photo-electron to be released. The photo-electron drifts toa region of high electric field where it accelerates and causesadditional electrons to be released by impact ionization. During thisbreakdown, current flows through device as the junction discharges tothe breakdown voltage. At this point, the junction recovers and againbegins to function as reverse biased diode. During the followingrecovery phase, current flowing through the device charges the junctionback to the bias voltage V_(b). During a complete breakdown and recoverycycle, the net amount of charge released from the microcell is:

Q=(C _(d) +C _(q))(V _(b) −V _(brk))  Equation 1

where C_(d) is the diode junction capacitance, and C_(q) is theparasitic capacitance across the quench resistor.

A scintillator (not shown) attached to the SiPM sensor 108 converts ahigh energy gamma-ray to many photons. The photons cause multiple cellsto breakdown. Since the cells of an SiPM sensor 108 are biased beyondV_(brk), they also breakdown randomly at a high rate due to thermaleffects, causing dark-current pulses.

Since dark pulses appear identical to the first photoelectron from atrue event, the high rate of dark-pulses can limit the performance ofthe discriminator. In some cases, a small subset of microcells maycontribute to most of the dark-pulses. In this case, others reduce theeffect of noisy microcells by deactivating them, requiring a uniquelook-up table for each SiPM sensor. However, this is not an idealsolution since the remaining microcells still create dark pulses, and itis cumbersome to have a unique look-up table for each SiPM sensor.

There is a need in the art for a design that does not unnecessarilydeactivate pixels to reduce false triggers due to dark pulses.

SUMMARY

Embodiments of the present invention generally relate to computedtomography and more specifically to discriminators and more specificallyto apparatuses, methods, and computer-readable mediums for minimizingthe effects of dark-current pulses. For example, in one embodiment ofthe invention, a method is provided where a first pixel is struck (i.e.,a primary pixel). Pixels struck within a fixed time frame after theprimary pixel is struck are referred to as “secondary pixels.” After ashort fixed time frame has expired (e.g., about 1 ns to about 5 ns), thenumber of primary and secondary pixels is added. If the count exceeds athreshold, it is clear the primary pixel was activated by the first (orearly) photon from a true gamma event. If the threshold is not met, thenit is likely the primary pixel generated a dark pulse that should beignored.

Other embodiments of the invention are provided that include apparatusesand methods having features similar to the method described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate only oneembodiment of this invention and is therefore not to be consideredlimiting of its scope, for the invention may admit to other equallyeffective embodiments.

FIG. 1 depicts a prior art scanning system

FIG. 2 depicts a prior art sensor;

FIG. 3 depicts close coupling of the electronics with a SiPM sensor;

FIG. 4 depicts a high level block diagram of an exemplary processingchain in accordance with embodiments of the invention;

FIG. 5 depicts an exemplary discriminator timing sequence in accordancewith the embodiments of this invention and demonstrates the operatingprinciple of the discriminator;

FIG. 6 depicts an exemplary bonded circuit in accordance withembodiments of the invention;

FIG. 7 depicts exemplary cell modules that interface to a sensor pixelin accordance with embodiments of the invention;

FIG. 8 depicts an exemplary pixel module in accordance with embodimentsof the invention;

FIG. 9 depicts an exemplary DLL in accordance with embodiments of theinvention;

FIG. 10 depicts an exemplary event control module in accordance withembodiments of the invention;

FIG. 11 depicts exemplary block adder/comparator in accordance withembodiments of the invention;

FIG. 12 depicts an exemplary pixel control state machine in accordancewith embodiments of the invention;

FIG. 13 depicts an exemplary top-level architecture for a digitaldiscriminator in accordance with embodiments of the invention; and

FIG. 14 depicts an exemplary method in accordance with embodiments ofthe invention.

To facilitate understanding, identical reference numerals have beenused, wherever possible, to designate identical elements that are commonto the figures.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toprovide a more thorough understanding of the invention. As will beapparent to those skilled in the art, however, various changes usingdifferent configurations may be made without departing from the scope ofthe invention. In other instances, well-known features have not beendescribed in order to avoid obscuring the invention. Thus, the inventionis not considered limited to the particular illustrative embodimentsshown in the specification and all such alternate embodiments areintended to be included in the scope of the appended claims. Althoughembodiments of the invention can be used in a combined PET/MR system theinvention is not limited to these systems. For example, otherembodiments of the invention can be used in a combined PET/CT orstandalone PET.

Embodiments of the discriminator described herein generate a time-markwhen it detects the first (or early) photo-electron from a SiliconPhotomultiplier (“SiPM”), and avoids false triggering due todark-current pulses common to these types of devices. Embodiments of thediscriminator architecture includes a sensor divided into blocks whichcan be processed concurrently, to prevent the performance of a largesensor from being degraded. Logic in the digital discriminator operatesasynchronously to allow pixels that detect dark-current pulses to becleared and enabled more quickly than possible in a clocked system. Thediscriminator also generates a final count of microcells for energyestimation after qualifying the event as being due to a gamma-ray. Inaddition, counts can be generated at intermediate intervals to estimatethe decay time of a gamma-event as an aid in analyzing thedepth-of-interaction of the gamma event by a downstream process.Depth-of-interaction can be used to make timing corrections to furtherimprove timing.

Another advantage associated with embodiments disclosed herein is autilization of a fully digital approach to integrate light output from aregion, or block, that minimizes the effects of noise. As described infurther detail below, a time-mark is generated from the first (or early)photo-electron from an event, and qualified (as an actual gamma-event)after a statistically significant number of microcells have detectedadditional photons from the same event. In this way, dark-current pulsesare prevented from generating a time-mark without the need to deactivatenoisy pixels. This approach will improve the overall system throughputby preventing false trigger events due to dark pulses. However, it isappreciated that other embodiments of the invention do not precludedeactivating noisy microcells to further enhance system throughput.

The arrival time of a gamma-ray is based upon when charge from the eventcan first be reliably detected. The best timing can generally beachieved by detecting the first photo-electron from the gamma-event. Theleading edge of a pulse due to the first photo-electron from agamma-event is identical to a dark-current pulse. A discriminator is thefirst element in the electronics processing chain. The method describedherein improves throughput since it rejects the dark-current pulses, butstill detects the first photo-electron from the gamma-event. Thisimproves efficiency by saving downstream resources for processing truegamma-events when they arrive.

In various embodiments of the invention, the structure of the SiPMsensor and front-end processing is analog. In the embodiment describedherein, the structure of the SiPM sensor and the front-end processingare digital. In a hybrid approach, the same discrimination principledescribed in this embodiment is applied to groups of microcells in ahierarchical approach. Since a response to the many photons releasedfrom a detector spans a wide dynamic range, it can be more difficult todetect the first photo-electron in a purely analog system.

When the front-end processing is analog, a Leading-Edge Discriminator(“LED”) with a low threshold set above the thermal noise floor can beused to detect the first photo-electron. In addition, the area and powerconsumption of the analog channel are important design considerations.

In other embodiments of the invention, a fully digital front end can beused when individual microcells from the SiPM sensor are available. Forexample, FIG. 3 depicts an embodiment of a digital discriminator 300(aka bonded circuit 300) that includes a SiPM sensor 302 bonded directlyto an application specific integrated Circuit (“ASIC”) 304. Forillustrative purposes only, the SiPM sensor 302 is depicted as having a5×5 array of pixels 306. However, it appreciated that variousembodiments of the invention include a SiPM sensor 302 having adifferent size array of pixels 306 (e.g., an 8×8 array of pixels 306).In an alternative hybrid SiPM embodiment, groups of microcells aresummed together hierarchically, and provide a reduced set of inputs tothe discriminator. The same general principle is used to avoid falsetriggers due to dark pulses.

In the fully digital embodiment described here, since the SiPM sensor302 is bonded to the ASIC 304, a quench resistor (not shown) can bemoved from the SiPM sensor 302 to the ASIC 304 simplifying bothcomponents. This improves the packing efficiency of the SiPM sensor 302,and manufacturing compromises needed to integrate resistors and/ortransistors into the sensor are eliminated.

The fully digital discriminator 300 described here receives inputs fromeach microcell. However, an analog or hybrid approach can be implementedbased on the same principle. In a fully digital solution, all componentsof the processing chain can be integrated on the same ASIC, including aTime-to-Digital Converter (TDC).

FIG. 4 depicts a high level block diagram of an exemplary processingchain 400 in accordance with embodiments of the invention. Theprocessing chain 400 includes a discriminator 402, a TDC 404, and anenergy channel 406.

The discriminator 402 generates a time-mark logic pulse when agamma-event is detected. The timing of the first photo-electron mayprovide the best timing of an event. The discriminator 402 ignoresdark-current pulses, improving its throughput compared to otherimplementations used in the prior art.

When the discriminator 402 detects an event, the TDC 404 and energychannel 406 further process data. Since each (i.e., the TDC 404 and theenergy channel 406) requires processing time and consumes power,responding to dark-current pulses lowers efficiency. By ignoringdark-current pulses, the signal processing chain 400 can remain ready toprocess true gamma-events.

The TDC 404 assigns a digital time-stamp to the time-mark generated bythe discriminator 402. The energy channel 406 estimates the energy ofthe gamma-event by integrating charge from the SiPM pixels affected bythe event. One of the advantages of the digital embodiment describedhere is that estimation of the gamma energy can be done with the samecomponents in the discriminator, after the event has been verified tonot be a dark pulse. In an analog implementation, additional integrationof the current produced by microcells activated by the event is neededto estimate the overall energy of the event.

A true gamma-event will cause many microcells from the first pixelstruck (aka the “primary pixel”) and adjacent pixels (aka the “secondarypixels”) to discharge in a short time interval after the firstphoto-electron. If the photo-electron is followed closely by charge fromthe primary or secondary pixels, embodiments of the invention indicatethat it is likely that the first (or early) photo-electron came from agamma-event instead of a dark-current pulse.

In other embodiments of the invention, a sum of the charge from theprimary pixel and its secondary pixels can be used to determine whetherthe sum exceeds a threshold (indicative of an actual event whenexceeding the threshold or a dark-current pulse when not exceeding thethreshold).

FIG. 5 depicts an exemplary discriminator timing sequence 500 inaccordance with embodiments of the invention. This figure shows thegeneral principle of operation for an analog, digital, or hybridimplementation. The timing sequence 500 includes a “Y-axis” 502delineating a magnitude of a pulse waveform 506, an arming threshold508, a timing threshold 510, and logic levels for various signals: atrigger comparator (“Trigger Comp”) output 512, an arming comparator(“Arming Comp”) output 516, the delayed trigger comparator output“D(Trigger Comp)” 516, a time-mark 524; and an “X-axis” 504 delineatingtime. The pulse waveform 506 represents the charge accumulation from theprimary and secondary pixels in a fully digital implementation, or thenet charge generated by an analog SiPM.

The trigger comparator output “Trig Comp” 512 generates a logic pulse514 when the pulse waveform 506 exceeds the Timing Threshold 510. Theoutput “Trig Comp” 512 is delayed by T_(del) 520 to form the delayedoutput “D(Trigger Comp)” 516.

T_(del) 520 is chosen longer than the time required for the aggregatepulse waveform 506 to be formed and exceed the Arming Threshold 508.

The “Arming Comp” 516 generates a high logic pulse 518 when the pulsewaveform exceeds the arming threshold 508. The output of the Armingcomparator is applied to the D-input of a flip-flop.

In the case of a gamma ray, the delayed output of the trigger comparator“D(Trigger Comp)” 522 creates a positive going edge at the clock-inputof the flip-flop after the arming threshold 508 is exceeded. Hence, thedelayed output of the trigger comparator “D(Trigger Comp)” 522 clocksthe flip-flop after sufficient set-up time has elapsed. The flip-flop isset only when the magnitude of the waveform pulse 506 exceeds the armingthreshold 508. The output of the flip-flop creates the “Time-mark”(T_(m)) 524 when set, and is valid after an additional fixed clock-to-Q(T_(c→Q)) time delay 528. Since a dark-current pulse never exceeds“Arming Threshold” 508, it never changes the state of the flip-flop. Thedelay of the time-mark T_(m) 524 is equal to T_(del) 520 plus T_(c→Q)528. Since these are fixed delays, they cause no relative timing errorsbetween opposing detectors.

FIG. 6 depicts examples of non-overlapping blocks in a bonded circuit300 in accordance with a fully digital embodiment of the invention.Although the bonded circuit 300 includes an ASIC, for simplicity theASIC is not shown. In addition, for illustrative purposes, each of theSiPM sensors 302 is depicted as having a 5×5 array of pixels. However,it is appreciated that in other embodiments of the invention the SiPMsensor 302 can have a differently sized array of pixels (e.g., 8×8,10×10, 13×13, or more).

A pixel in which the first photo-electron is detected is referred to asthe primary pixel. In this embodiment of the invention, contributionsfrom secondary neighboring pixels are added to the primary pixel togenerate an arming signal to qualify it as a gamma-event. In variousembodiments of the invention, the contribution of primary and secondarypixels may also be summed to estimate the gamma energy after a longerintegration interval (e.g., about 50 ns to about 100 ns).

In the examples provided in FIG. 6 (i.e., examples 600, 604, and 608),the primary pixel in each block is shaded darker than the secondarypixels associated therewith. Three different types of blocks are shownand are categorized as: a Full block, an Edge block, or a Corner blockIn each of the examples 600, 604, and 608, pixels are identified by an(x, y) index. Each of the pixels has a plurality of microcells (notshown) configured in an m×m array.

Note that in each of the examples 600, 604, and 608 none of the primarypixels have the same secondary pixels during the same event time. As aresult, the primary pixels are considered non-lapping and concurrentanalysis of the block(s) can be performed.

Specifically, example 600 includes primary pixels 602 ₁₁ and 602 ₄₄.Primary pixel 602 ₁₁ is a corner block. Because primary pixel 602 ₁₁ isa corner block (because the primary pixel 602 ₁₁ is in the corner) thereare five secondary pixels (e.g., secondary pixel 602 ₂₁) that can bestruck to indicate that an actual gamma-event has occurred. Primarypixel 602 ₄₄ is a full block and has eight secondary pixels (e.g.,secondary pixels 602 ₃₅ and 602 ₅₃) that can be struck to indicate thatan actual gamma-event has occurred.

In example 604, primary pixel 606 ₃₃ is part of a full block (i.e.,having a primary pixel that is neither a corner pixel nor and edgepixel) and has eight secondary pixels (e.g., secondary pixels 606 ₄₂ and606 ₂₃) that can be struck if an event is an actual gamma-event.

In example 608, primary pixel 610 ₁₂ is part of an edge block (i.e.,because the primary pixel 610 ₁₂ is an edge pixel) and has fivesecondary pixels (e.g., secondary pixel 610 ₂₁) that can be struck toindicate that an actual gamma-event has occurred.

Primary pixel 610 ₄₄ is part of a full block and operates as explainedabove regarding primary pixel 602 ₄₄ in example 600.

Some of the advantages associated with dividing the SiPM sensor 302 intoblocks are: limiting the set of secondary pixels to a block around theprimary pixel to reduce the noise contribution from pixels outside theblock, which contribute little to the arming and/or energy signals; andnon-overlapping blocks can be processed independently to help preservethe efficiency of a large sensor array. Smaller blocks allow moreprocessing channels to work concurrently, allowing each detector toprocess more events.

The size and shape of various block structures are determined for thenumber of pixels in the sensor based on light output simulations of thedetector.

When adequate resources are available, the electronics, ASIC can processmultiple blocks concurrently. This isn't difficult using an ASIC havinga small feature size (e.g., 0.18 micro, 0.13 micro, 65 nm, etc.).

FIG. 7 depicts an exemplary 700 cell module and interface to a sensorpixel (i.e., an m×m microcell pixel 702) in accordance with the fullydigital embodiment of this invention. There is provided a contact foreach microcell 702 on the sensor 302 that allows a quench resistor 706to be moved to the electronics ASIC (i.e., on a respective Cell Module704).

A respective Cell Module (Cell Module 704 ₁, . . . , Cell Module 704_(m) (collectively Cell Module 704)) is connected to a respectivemicrocell (microcell 702 ₁, . . . , microcell 702 _(m) (collectivelymicrocell 702)), as shown in FIG. 7. For example, Cell Module 704 ₁ isconnected to microcell pixel 702 ₁.

Each Cell Module 704 includes a quench resistor 706 (quench resistor 706₁, . . . , quench resistor 706 _(m) (collectively quench resistor 706)),a transistor 708 (transistor 708 ₁, . . . , transistor 708 _(m)(collectively transistor 708)), another resistor 710 (resistor 710 ₁, .. . , resistor 710 _(m) (collectively resistor 710)), and a flip-flop712 (flip-flop 712 ₁, . . . , flip-flop 712 _(m) (collectively flip-flop712)).

The junction of the transistor 708 is recharged faster than the timerequired to determine whether an event is a dark-current pulse, foroptimal performance. The quench resistor 706 allows the transistor 708to recharge faster. In other embodiments of the invention, an activequench circuit is used (in place of the quench resistor 706).

The transistor 708 (e.g., a fast bipolar transistor or a CMOS device)provides a sharp transition to the clock port of the flip-flop 712.

The flip-flop 712 is in an enabled state while waiting for an event. Theflip-flop 712 is set when the microcell 702 discharges.

The voltage at the anode of all microcells 702 in a pixel is adjustedthrough each quench resistor 706 (or other active circuit) by adigital-to-analog converter (“DAC”) 714. The DAC allows fine gainchanges of each pixel to compensate for variations in process, voltage,and temperature (“PVT”), and can be adjustable over a small range (about1 volt) in small steps.

During operation, the flip-flop 712 is set when a microcell 702discharges, and stays set until it is cleared. The quench resistor 706stops the breakdown of the cell 702, and allows it to recharge quickly.The cell 702 recovers before the time period needed to determine whetherthe discharge was due to a dark-current pulse, so it can be returned tooperation with a minimum dead-time. Until the Cell Module 704 is reset,subsequent breakdown of the cells 702 is ignored, since the flip-flop712 is not cleared until the event processing is complete. It isunlikely photons from the same gamma-event will trigger the samemicrocell more than once. Breakdowns within the time frame of thecurrent event are more likely due to additional dark-current pulses.Each cell module 704 is connected to the digital-analog converter 714(“DAC”) to allow fine control of the SiPM anode bias voltage.

FIG. 8 depicts an exemplary Pixel Module 800 in accordance withembodiments of the invention. The Pixel Module 800 is the processingelement for each pixel. The Pixel Module 800 receives input from eachflip-flop 712 in the Cell Module 704 (for simplicity the Cell Module isnot depicted in FIG. 8).

Each Pixel Module 800 includes a register/adder module 804, an OR-Logicmodule 806, a delay-line 808, and a flip-flop 810.

The register/adder module 804 captures the number of Cell Modules (notshown) that are set (via a respective flip-flop 712), and then generatesa sum. The register 812 captures the state of all the Cell Modules inthe pixel on the rising edge of cntEn 818. Since the inputs to theregister 812 are asynchronous to the clock signal from the flip-flop712, there is some risk of metastability.

Metastability may cause one or more bits to briefly oscillate beforesettling, which can cause counting errors. This condition can bevirtually eliminated by allowing adequate time for the register 812 tosettle before reading the output of the adder 814, double registering,and using well designed registers with a small set-up and hold time. Asmall number of errors will not have any significant effect on theoutcome of the calculation. In this particular digital implementation,the contents of the adder 814 will be captured twice. The first countwill be used to qualify if a pulse was due to a gamma-event by countingthe number of cells set in the primary pixel, and optionally adding thesum of all secondary microcells set in the block. If the total countexceeds a threshold, a gamma-event will have been detected. In thiscase, the discriminator 300 will issue the time-mark pulse tm 526. Thecounting process will then continue for an extended period to allow thepixel module 800 to count all the photons in the block for the event.The total number of microcells set after this extended counting periodwill provide a measure of the energy of the gamma-ray. This longer termcount can be used to further qualify the gamma energy.

The Or-Logic block 806 produces a logic pulse when any microcell in thepixel is set. To generate at time-mark quickly, fast bipolar transistorsshould be used, and capacitance minimized by using the smallesttransistors possible (with high transconductance). In other embodimentsof the invention, a hierarchical triggering approach is utilized toreduce the number of connections between the SiPM array anddiscriminator. The output of this block generates a “start” signal 820to the controller (not shown).

The Delay-line 808 delays the “start” signal 820 from the Or-logic block806 by a fixed time delay before the time-mark tm 526 is generated bythe pixel module 800. This time mark is issued a fixed delay after thefirst flip-flop 712 is set, corresponding to the first photoelectronbeing detected by the pixel module 700. The length of this time delay isdetermined by the delay line 808, and corresponds to T_(del) 520. Asdescribed in the analog embodiment of the invention 500, this delay mustbe long enough to enable the sum of all microcells in the current pixel(or block) to be determined If the event is valid, the controller willassert the tmEn signal 822 flip-flop 810 to be set. The flip-flop 810 isclocked by the delay line 808 to generate the time-mark input to the TDC404 (not depicted in FIG. 8). The TDC 404 should clear the flip-flop 810soon after it is detected by pulsing tmClr 824. If the event was not dueto a gamma-ray, the controller will mask the D-input of the flip-flop810. The “end” signal 826 informs the controller that the delay line 808has been flushed. The delay line 808 implementing T_(del) is a resourcebelonging to each pixel module. Since every pixel has the resources togenerate a time-mark, multiple pixels can operate concurrently if theydon't use secondary pixels already in use by another block.

The delay-line 808 is used for timing, and is optimized for low jitter,and longer term variations due to voltage and temperature. Fixed processvariations are not important since they can be calibrated out.Variations in the delay cells can be minimized by a Delay-Lock-Loop(“DLL”) (described below and depicted in FIG. 9).

FIG. 9 depicts an exemplary DLL 900 in accordance with embodiments ofthe invention. In the DLL 900, a precision reference clock 904 suppliesa signal to a reference delay line 908. The length of the delay-line 908is approximately equal to the period of the clock. The phase between thereference clock 904 and the output of the delay line 908 is compared toproduce an error signal. The loop-filter 902 applies a gain and limitsthe bandwidth of the error signal to generate a correction added to anominal bias voltage. The corrected bias voltage compensates the delaycells in 908 for voltage and temperature variations. A verylow-bandwidth loop filter reduces jitter. The same bias voltages canthen be used to compensate other critical delay lines in the design,such as used in the pixel modules 808. Multiple DLLs can be useddepending on the size of the electronics ASIC (not shown). In this way,all delay lines are calibrated using 908 as a reference.

The controller also uses delays that can be stabilized by the delay line908, although delays required by the controller do not need to be asprecise. For example, taps in the controller delay will be used forinternal timing signals

Taps in the delay-line 808 in the pixel module 800 will not be used forcontrol, to avoid introducing jitter.

FIG. 10 depicts an exemplary Event Control Module 1000 in accordancewith the fully digital embodiment of the invention. The Event ControlModule 1000 includes n×n segments, one corresponding to each PixelModule 1002 (illustratively depicted in FIG. 10 as segment 1002 ₁, 1002₂, . . . , 1002 _(n) (collectively referred to herein as Event ControlModule segments 1002)); logic circuitry 1004 (illustratively depicted inFIG. 10 as logic circuitry 1004 ₁, 1004 ₂, . . . , 1004 _(n)(collectively referred to herein as logic circuitry 1004)); flip-flop1006 (illustratively depicted in FIG. 10 as flip-flops 1006 ₁, 1006 ₂, .. . , 1006 _(n) (collectively referred to herein as flip-flop 1006));and flip-flop 1008 (illustratively depicted in FIG. 10 as flip-flops1008 ₁, 1008 ₂, . . . , 1008 _(n) (collectively referred to herein asflip-flop 1002)).

When a primary Pixel Module detects a microcell has been triggered, itwill request processing by the Pixel Control Module. A state-machine(not shown) in the Pixel Control Module will then supply timing signalsto both primary and secondary pixels in the block. The Event ControlModule 1000 decides if a pixel should process an event or ignore it. Toprocess the event, the secondary pixels in the block must be availablefor counting pixels. If one or more secondary pixels are not available,the event will be ignored. Once processing of an event has started, allpixels in the block will remain in a busy state until processing of theevent is complete. While the block is busy, other primary pixels thatare triggered, and would use one or more secondary pixels that arealready busy, will ignore those events. In principle, only one primarypixel is allowed to control a secondary pixel in its block during thetime period required to process the event.

One segment of the Event Control Module 1000 is reserved for each pixel1002. The start signal from each pixel clocks two flip-flops (i.e.,flip-flop 1006 and flip-flop 1008) in the segment corresponding to thepixel. The states of flip-flop 1006 and flip-flop 1008 are inputs to thePixel Control Module state-machine (not shown).

Flip-flop 1006 signals the Pixel Control Module that an event has beendetected by the pixel module 1002. The output of flip-flop 1008 dependson the state of the registers (not shown) in the Event Control Module1000. Flip-flop 1008 is set only if the secondary pixels that would beneeded by the block are available. This is determined by logic 1004,which is a function of the primary pixels that are currently busy. Thebusy pixels are maintained by the state of the registers in the eventcontrol module 1000. If the secondary pixels are not available, thepixel control module 800 will ignore the event, and reset the primarypixel. If the secondary pixels are available, the state-machines takecontrol of the primary and secondary pixels to process the event.

Since the start signal 820 from each Pixel Module block arrivesasynchronously, there is a small probability that a conflict will not beprevented, leading to an occasional counting error. Fast event controlmodule logic will minimize this risk. Infrequent errors will not have asignificant impact on the efficiency of the discriminator. The effect ofthis error is similar to gamma-rays that deposit energy in adjacentdetectors. In both cases, the event data will be ignored since it willnot pass the energy qualification.

Logical expressions are needed to determine if a primary pixel canprocess events. A block is defined for each primary pixel, and definesthe secondary pixels needed to determine if the event is due to agamma-ray. The set of all pixels that are currently busy is a functionof the primary pixels that are currently processing events, and theirsecondary pixels.

Methods for deriving a logical expression for a large detector array canbe described. For example, returning to the 5×5 detector 300 depicted inFIG. 6, let the ordered pair of integers (x,y) identify the location ofeach pixel. Assume three block geometries are defined for this sensor,as shown in FIG. 6. The block types are Corner (“C”), Full (“F”), andEdge (“E”). More elaborate shapes can be determined for larger detectorsbased on light-output studies. Table 1, below, lists each primary pixelon the left, with the block shape in column T (type). Note that thetable demonstrates a simple repetitive pattern that can be extrapolatedfor larger sensor arrays.

TABLE 1

The pixel locations are also displayed in columns across the top. A “1”appears in the x-y grid if the pixel in the column is secondary for theprimary pixel in the row. Pixels along the diagonal (dark-gray)correspond to the primary pixel. In the simple example, TABLE 1 isanti-symmetric relative to the center row and column (light-gray), dueto the symmetry of the detector. For example, the last row is identicalto the first row, but in reverse order. Similarly the last column isidentical to the first column, again in reverse order.

TABLE 2 can be derived from TABLE 1.

TABLE 2

For example, to derive the list of pixels that would conflict withprimary pixels (3,2):

1. Find the row in TABLE 1 corresponding to primary pixels (3,2);

2. Columns in this row marked “1” are the secondary pixels for theprimary pixels (3,2). For each column containing a “1” (in this row),consider all other rows in this column. If any row contains a “1,” markthe primary pixel (on the left) with a “1.”

3. Repeat step 2 for the remaining columns in the row corresponding toprimary pixel (3,2), marking the primary pixels as above. Pixels on theleft may be marked multiple times.

4. Any pixel on the left of TABLE 1 marked with a “1” conflicts withpixel (3,2). This list of marked pixels becomes the row for pixel (3,2)in TABLE 2.

Note that TABLE 2 can be used to derive the logic functions used by theevent control module. This table is also anti-symmetric relative to themiddle row and column, similar to TABLE 1. This demonstrates that TABLE2 can be generated by a simple repeatable process, and extended tolarger sensor arrays.

When the primary pixel is active, counts of microcells are generated atleast twice.

The first count determines whether enough microcells have been set aftera short time delay to determine if the event was due to a gamma or adark-current pulse. In some embodiments of the invention, this count isdue to the number of primary cells set. In other embodiments of theinvention, the count is due to the sum of primary and secondary cells inthe block. Determination of which embodiment to use can be determined bysimulating the light output of the detectors.

A second count may be used to determine total number of cells set in theblock after a longer integration interval, and can be used to estimatethe gamma-energy. This count will occur during an extended period onlyafter a gamma-event is detected. The counting period is long enough formost of the light to be collected from the detector. This feature is nota requirement for the discriminator to reject dark pulses, but is aconvenience since the resources are available to generate photon counts.

Additional cell counts can also provide information about the rate atwhich the light output from the detector is decaying. This is useful forestimating the depth of interaction in the detector.

FIG. 11 depicts exemplary block adder/comparators 1100 and 1102 inaccordance with the fully digital embodiment of the invention. Blockadder/comparator 1100 forms the sum of all cells in the block todetermine the type of event that occurred. Block adder/comparator 1102uses the cells in the primary pixel. In block adder/comparators 1100 and1102, the valid signal is an input to the pixel control module, and isset to logic “1” when the output exceeds the arming threshold 1108. Theenergy out 1106 is generally formed by adding the number of cells set inthe primary and secondary pixels in the block, and is optionally usedfor an energy qualification. The same arming threshold 1108 is common toall pixels, and performs the equivalent function 508 in FIG. 5. A blockadder/comparator 1100 and block adder/comparator 1102 are allocated foreach pixel, with inputs from the primary and secondary pixels in theblock.

Since this part of the system is not clocked, there is no speedadvantage for a serial adder. A parallel adder will use fewer gates,since no intermediate registers are needed for pipe-delays. A fast-carrylook-ahead adder can be used to provide a sufficient time delay for theoutput to settle. The block adder/comparators 1100 and 1102 can be moreefficient by comparing just the high bits, since precision is notcritical. Pipe delays in the block adder/comparators 1100 and 1102 canreduce power consumption by lowering the number of bits that changestate as the output settles.

FIG. 12 depicts an exemplary Pixel Control State Machine 1200 inaccordance with the digital embodiments of the invention. Each PixelModule 800 has an independent Pixel Control Module state-machine 1200.When a Pixel Module 800 detects an event, it generates a start signal.Based on the status of other pixels, the Event Control Module 1000asserts one or both enable signals en1 1202 and en2 1204 to thestate-machine 1200. The state-machine 1200 will then send controlsignals to resources in the cell modules 704 and pixel modules 800 todetermine if the triggering event was a dark-current pulse or agamma-event. If the triggering event is a dark-current pulse, the pixelis made ready to detect the next event as quickly as possible. If thetrigger was due to a gamma-event, the controller 1200 may optionallycontinue to process the block to obtain a count of all microcells set inthe block at the end of the event for the energy qualification.

The discriminator 402 responds to asynchronous events, and recovers fromdark-current pulses as quickly as possible to maximize throughput. Thisis one reason that the discriminator is not clocked. This eliminates theneed to synchronize signals to or from a clock. Level-mode digitalsystems rely on gate and routing delays to ensure predictabletransitions between machine states. In FIG. 12, one or more delay-linescan be sufficient to synchronize transitions between states. If agamma-event is detected, a pulse through a longer delay-line can be usedto determine the integration time in which the total photon count iscollected for the optionally energy qualification. Since various taps onthese delay lines drive various gate loads, timing cannot be maintainedas precisely as in the case of an unloaded delay-line 808. This is why aseparate delay line 808 is used to generate the time-mark in the pixelmodule 800.

Although the exemplary state-machine 1200 is described herein, thatdescription is for illustrative purposes only. It is appreciated thatother state-machines can be used in accordance with the invention.

The illustrative state-machine 1200 works as follows:

Enable signals en1 1202 and en2 1204 are inputs from the Event ControlModule 1000 and indicate that a pixel has detected an event. If bothenable signals 1202 and 1204 are set, the secondary pixels in the blockare available so the event can be processed. If only one enable signalis set, the secondary pixels are busy, and the event should be ignored.The pixel can be cleared after the time-mark has been flushed from thedelay line 808 in the Pixel Module 800. If both enables 1202 and 1204are set, other primary pixels will be prevented from processing eventsby the Event Control Module 1000, that need to share secondary pixels.

When the pixel is enabled to process an event, the controller will waitfor a delay corresponding to T_(del) 520 before raising the cntEn signal818. In this digital embodiment, this delay is implemented by thedelay-line 808 in each pixel control module 800. The rising edge ofcntEn 818 will cause the register in the pixel module 800 to capture thestate of all microcells. The counter in each pixel module 800 willupdate, after which the block sum will update. The valid output of thecomparator will settle to its correct value. The controller will thenmake a decision to continue processing the pixel, or ignore it.

If the valid signal from the comparator 1100 indicates the event was adark-current pulse, it disables tmEn 822, preventing the time-mark tm526 from leaving the pixel module 800. The controller then waits for anend pulse to indicate that the time-mark has been flushed from thehigh-precision delay line in the pixel module 800. If a gamma-event wasdetected, the controller 1200 asserts tmEn 822 allowing the flip-flop810 to be set, creating the time mark 526. Optionally, the controllerwaits for a longer delay to allow the block sum to update after all (ormost) of the photons from the detector have been collected. Thecontroller 1200 will raise cntEn 818 again to count the total number ofcells set due to the current gamma event. After another delay to allowthe block count to settle, the controller 1200 strobes cntRdy 1208 tosignal down-stream hardware that the final energy count is valid. Theblock count can be captured by a register on the rising edge of cntRdy1208. This process to estimate the total gamma in the energy isoptional.

After the event has been processed, clrPixel 12010 and clrEvent 1212pulses to clear registers in the Event Control Module 1000 and CellModules 704, making the pixel ready to capture the next event. If theevent was a dark-current pulse, the pixel will become active after theminimum time needed to determine the type of event. The ControlState-Machine 1200 controls the primary and secondary Cell Modules 704and Pixel Modules 800 in the block. Since every pixel can function as aprimary or secondary pixel, control signals will be OR functions fromseveral pixel control module state-machines 1200. Errors in whichmultiple state-machines attempt to control the same pixel can occur, butwill be rare. These events will likely fail the energy qualificationtest, and be ignored.

FIG. 13 depicts an exemplary top-level architecture 1300 for a fullydigital discriminator in accordance with embodiments of the invention.The top-level architecture 1300 includes the Cell Modules 704, PixelModule 800, Pixel Control State-Machine 1200, Event Control Module 1000,and adder/comparator(s) 1100 and 1102. The outputs are: A time-mark tm526 to the TDC; and a count of the number of microcells (blkCnt) 1302set in the block during an integration interval in which all (or most)of the photons from the detector should have been collected. The data isvalid when the cntRdy 1208 output is asserted.

FIG. 14 depicts an exemplary method 1400 in accordance with embodimentsof the invention. The method 1400 begins at step 1402 and proceeds tostep 1404.

At step 1404, a first microcell is struck by a photon. This firstmicrocell is one microcell in a microcell array of a pixel (typicallythe pixel includes about 3600 microcells). The number of pixels in ablock can vary (e.g., a 5×5 pixel array or an 8×8 pixel array). Thepixel containing the first microcell is referred to as the “primarypixel.” After the first microcell is struck, the method 1400 proceeds tostep 1406.

At step 1406, after a first time frame (e.g., about ins to about 5 ns)all of the microcells that were struck in the primary pixel and all ofthe microcells that were struck in the secondary pixels (associated withthe primary pixel) in the block are counted. As explained above, eachprimary pixel has multiple secondary pixels associated therewith. Aftercounting, the method 1400 proceeds to step 1408.

At step 1408, the count (i.e., number) of the struck microcells iscompared to a threshold number.

At step 1410, a determination is made whether the count exceeds thethreshold. If the count exceeds the threshold then it is determined thatthe first microcell was struck by a photon due to a gamma-event.However, if the count does not exceed the threshold then it isdetermined that the first microcell generated a dark pulse and isignored. Note that ignoring the striking of the first microcell can bedone without deactivating microcells. If a negative determination ismade a step 1410, the method 1400 proceeds to and ends at step 1412.

If however it is determined at step 1410 that a gamma event hasoccurred, the method 1400 proceeds to step 1414. At step 1414 along-term (e.g., about 50 ns to about 100 ns) count of all primary andsecondary pixels is collected to estimate the energy content of thegamma This additional information is provided to down-stream processes.

While the foregoing is directed to a fully digital embodiment of thepresent invention, other and further embodiments of the invention may bedevised without departing from the basic scope thereof, and the scopethereof is determined by the claims that follow.

We claim:
 1. A method comprising: accumulating a charge from primary andsecondary pixels; generating a first logic pulse when said accumulationexceeds a timing threshold, wherein said first logic pulse is delayed bya first time period to form a delayed output, and said first time periodis greater than a formation time of said accumulation and exceeds anarming threshold; generating a high logic pulse when said accumulationexceeds said arming threshold; and applying said high logic pulse tosaid delayed output.
 2. The method of claim 1 wherein said delayedoutput is a positive going edge for a flip-flop clock-input, of aflip-flop, after said arming threshold is exceeded.
 3. The method ofclaim 2 wherein output of said flip-flop creates a time-mark, whereinsaid time-mark is valid after an additional fixed time delay.
 4. Themethod of claim 3 wherein said time mark is equal to said time mark plussaid additional fixed time delay.